Two-way satellite communication systems transmit and receive data in various frequency bands. For example, some systems operate in the Ka-band, which is between about 17 and 36 GHz. Other systems operate in bands such as the C-band (3.7–6.4 GHz) or the Ku-band (11–15 GHz), for example. Future systems may use higher frequencies (e.g., 60 GHz).
Modulation and upconversion are essential methods used in two-way satellite communication systems and in other wireless communication systems. Upconversion is the translation of a signal's frequency from baseband, or the original frequency before modulation, to a higher frequency. The signal is then transmitted at this higher frequency. Upconversion is performed because most antennas can only receive signals that have short wavelengths. Frequency is the inverse of wavelength. Therefore, the higher the frequency a signal has, the shorter its wavelength. Consequently, signals upconverted to a higher frequency are easier to transmit.
Modulation is a method used to transmit and receive data using a carrier signal. Modulated signals can be analog or digital signals. By varying the phase of a digital carrier signal, for example, information can be conveyed. This type of modulation is called phase-shift keying (PSK). There are several schemes that can be used to accomplish PSK. The simplest method uses only two signal phases: 0 degrees and 180 degrees. The digital signal is broken up time wise into individual bits (binary digits—zeros and ones). The state of each bit is determined according to the state of the preceding bit. If the phase of the wave does not change, then the signal state stays the same (low or high). If the phase of the wave changes by 180 degrees-that is, if the phase reverses-then the signal state changes (from low to high, or from high to low). Because there are two possible wave phases, this form of PSK is sometimes called Binary Phase Shift Keying (BPSK).
A more complex form of PSK is called Quadrature Phase Shift Keying (QPSK). QPSK modulation employs four wave phases and allows binary data to be transmitted at a faster rate per phase change than is possible with BPSK modulation. In QPSK modulation, the signal to be transmitted is first separated into two signals: the In-phase (I) signal and the Quadrature (Q) signal. The I and Q signals are orthogonal, or 90 degrees out of phase. Thus, they are totally independent and do not interfere with each other. Each signal can then be phase shifted independently. Both the I and Q signals have two possible phase states. Combining the possible states for the I and Q signals results in four total possible states. Each state can then represent two bits. Thus, twice the information can be conveyed using QPSK modulation instead of BPSK modulation. For this reason, QPSK modulation is used in many two-way satellite communication systems.
Currently, upconversion in most two-way satellite communication systems entails a multi-stage conversion process. First, baseband QPSK I, Q streams are modulated and then upconverted to an Intermediate Frequency (IF) (e.g., 1.7–2.2 GHz). This conversion is performed by in an Indoor Unit (IDU). The signal is then upconverted again to a transmit frequency, fTX (e.g., 29.5–30.0 GHz), in an Outdoor Unit (ODU) located at the terminal's antenna. The upconversion is then complete and the signal is ready for transmission.
The output signal (transmit signal) of the ODU has associated with it a certain power level. The ODU output signal power level is regulated in many countries and cannot exceed certain levels. The maximum allowable output signal power level varies by country.
In many two-way satellite communication systems, for example, the optimal output signal power level of the ODU is 4 watts. Some countries allow this output signal power level. However, other countries are more limiting in their regulations and allow ODU output signal power levels of no more than 2 watts, for example.
A high ODU output signal power level is preferable to a low ODU output signal power level because the higher output signal power level is easier to detect and receive. A high ODU output signal power level requires a smaller receiving antenna than does a small ODU output signal power level. Small antennas are usually easier and more cost-effective to design and construct than are large antennas.
The same various limitations on transmitter output signal power levels could be imposed on any transmitting device used in wireless communication systems. Thus, as used hereafter and in the appended claims, the term “two-way satellite communication systems” will be used to refer expansively to all possible two-way satellite communication systems and other applications where the maximum output signal power level of a transmitter is regulated. In addition, the term “ODU” will be used to refer expansively to all possible transmitters.
Thus, there is a need in the art for a method and system of limiting the ODU output signal power level to various levels so that it is always equal to the maximum allowable power level depending on the country within which the two-way satellite communication system operates.
There have been several approaches to complying with the various ODU output signal power level restrictions. One solution is to fix the ODU output signal power level to equal the lowest maximum allowable output signal power level of the countries within which the two-way satellite communication system might operate. For example, if the lowest maximum allowable output signal power level is 2 watts, the ODU amplification circuitry could be modified so that the maximum power level of the output signal never exceeds 2 watts. This ODU output signal power level is obviously not optimal in the countries with higher maximum allowable output signal power levels.
Another traditional solution to limit the ODU output signal power level to various levels is to use Automatic Gain Control (AGC). AGC is a process or means by which the gain (output power versus input power) of the ODU is automatically adjusted as a function of a specified parameter, such as the output signal level. However, AGC cannot be used in the ODU of many two-way satellite communication systems because it takes too long to lock into the desired gain. Also, the ODU output signal power level needs to change according to varying weather conditions. It is currently difficult, if not impossible, for an AGC circuit to adjust for varying weather conditions.
Another possible solution is an IDU that is capable of adjusting the ODU input signal level power. This requires a means for calculating the ODU gain and an interface unit for communicating this gain information to the IDU. The IDU would then need to adjust the ODU input signal power level based on this gain information so that the ODU output signal power level can change to the desired level. However, this process is currently limited by the speed at which the interface unit between the IDU and ODU operates and is therefore too slow for many applications. In addition, it requires an IDU capable of adjusting the input signal level power of the ODU. This capability might not be present in many systems.
Another possible solution that has been explored is to monitor the direct current (DC) current of the output signal of the ODU. Then, according to the monitored DC current, the IDU varies the output signal level which is input into the ODU to adjust the power level of the output signal.
For example, if the power level of the output signal is desired to be less than 2 watts, but it is currently higher than 2 watts, then the DC current of the output signal is higher than it would be at the desired power level. Reducing the ODU input signal power level decreases the ODU output signal power level as well as the DC current of the output signal.
However, in many two-way satellite communication systems, it is difficult to correlate the DC current and the radio frequency (RF) output signal power. This is, in part, due to the use of a class-A wideband power amplifier (PA) in the ODU. Class-A PAs are used because they reproduce the input signal with little distortion. They are, however, the least efficient among the different classes of PAs because the power of their output signals is only a small percentage of the DC power used in the amplification process. The degree of inefficiency varies from PA to PA and thus, the correlation between the DC current of the output signal and its power level is unpredictable.